Venus transit: A last-minute guide

Thousands of scientists and skywatchers around the world have made detailed plans to monitor today's transit of Venus across the sun, but chances are that word of the last-in-a-lifetime event is just now sinking in for millions of just plain folks — so what's the big deal? And what's the best way to watch the transit?

We've had dozens of stories about Venus' day in the sun over the past few weeks, but for those of you who are just tuning in, here are the top 10 things to keep in mind about today's transit, whether your skies are sunny or completely socked in:

1. Get the big picture: Venus comes between Earth and the sun five times in the course of every eight years, but because of the inclination of the planets' orbits, Venus usually misses passing over the sun's disk, as seen from Earth. In fact, that passing-over phenomenon occurs only twice in the typical person's lifetime. Two transits occur eight years apart, but each pair is separated by either 105.5 years or 121.5 years. We had a Venus transit in 2004, and we're having another one today. The next one won't come until 2117. So if you're into rare sky phenomena, today is as good as it gets.

2. Find out when and where: Venus' disk begins to pass over the left edge of the sun's disk a little after 6 p.m. ET, and makes a stately crossing that lasts until about 12:50 a.m. ET. (Of course, the sun will have set on the East Coast by then.) Some part of the transit will be visible from most locations on Earth — though you're out of luck if you're in eastern South America, western Africa, the Iberian Peninsula, Antarctica or the middle of the Atlantic.

The precise time when different edges of the planet's disk cross the sun's edge is actually a big deal. Those times vary by location on Earth, and the variations can be used to calculate dimensions and distances in the solar system. Today, so much is known about those dimensions that astronomers can predict the key times of the transit based on your location. To find out what you can see when, use the U.S. Naval Observatory's transit computer.

3. Safety first: This is where I type out the bold-face warning that you should never gaze at the sun without proper eye protection. Sunglasses are not adequate. Neither are black plastic garbage bags, or film negatives. Unsafe viewing can damage your retinas. This video from "Astronomy Dave" Fuller explains the difference between proper and improper eye protection, not only for the transit, but for anytime you want to observe the sun:

Here's how to see the sun safely for transit viewing or sunspot viewing.

4. Use solar-filter glasses: If you've been looking forward to this transit, or if you observed the annular solar eclipse a couple of weeks ago, you probably already have your special sun-viewing glasses. But if you don't, you still might be able to find the glasses at a science-center gift shop, observatory or camera shop. Sheets of solar-filter material may be available at specialty shops, wherever fine telescopes are sold — but you should make sure that the material is considered safe for visual observations. No. 14 welder's glass also serves as a suitable solar filter. But No. 13? Not so much.

Because Venus' disk is only 3 percent as wide as the solar disk, it can be challenging to make out the dark spot without magnification. Paul Doherty, a staff physicist at San Francisco's Exploratorium, makes this comparison: Take a 7-inch-wide (15-centimeter-wide) paper plate, and draw a black dot on the plate that's less than a quarter-inch wide (5 mm). Then tack the plate onto a wall and back up 45 feet (15 meters). That black dot is what Venus will look like as it passes over the sun.

6. Use binoculars or telescopes: You can use magnifying devices to get a bigger picture of the transit, but you have to know what you're doing. The best method is to put a specially made solar filter over the front aperture of your telescope, or over each of the front lenses of your binoculars. (You can also use a filter on one binocular lens and tape over the other lens to shut out the sunlight.) Some folks have carefully taped filter material from sun-viewing glasses over the front lenses of binoculars. But it's not safe to look through a telescope, viewfinder scope or pair of binoculars without a filter on the front end, even if you're wearing sun-viewing glasses. That's because sunlight will be concentrated by the instrument's lenses and potentially burn through the filter or even crack welder's glass.

7. Find an astronomer: You'll have the best experience if you're with a group that includes a knowledgeable amateur or professional astronomer. Science centers and national parks are likely to be hotspots for transit-viewing parties. Your local astronomy club is probably partying it up as well. Check out this worldwide directory of astronomy clubs, or this listing from the Astronomical League. If nothing else, call up the physics department of the closest college or university and find out what's going on.

8. See it on the Web: I'm guessing that most transit-watchers will be getting at least some of their looks via the Internet, particularly if the weather is lousy. Here's a listing of the webcasts that'll be available, including msnbc.com's simulcast of NASA's coverage:

Exploratorium: The San Francisco science center's seasoned webcast team will be webcasting from Mauna Loa, Hawaii, starting at 6 p.m. ET.

Univ. of North Dakota SEMS (in Alaska): UND's Tim Young says the road show and the chat will start cooking from Alaska at 5:45 p.m. ET. "It is one of two locations in the U.S. that will see the whole transit," he told me via email. "The other is Hawaii, and other groups are webcasting from there."

Slooh Space Camera:Slooh starts its rock-solid webcast at 5:30 p.m. ET, featuring a dozen or more video feeds from Sweden, Norway, Australia, New Zealand, Hawaii, Arizona, New Mexico and other locales.

Universe Today: Live shots from around the world with commentary from Bad Astronomer Phil Plait, Pamela Gay and other celebrity guests.

Coca-Cola Space Science Center: Columbus State University's science center in Georgia is offering pictures from the home base in Columbus, Ga., as well as from Utah, Mongolia and Australia. The webcast gets started at 5:30 p.m. ET.

9. Soak in the science: Since the first transit of Venus was predicted in the 17th century, scientists have been taking advantage of the phenomenon. As I mentioned previously, measurements of the transit timing have been used to derive highly accurate estimates of the solar system's scale. Nowadays, transit observations play a key role in detecting and confirming the presence of planets around other stars. (NASA's Kepler planet-hunting mission has a Web page devoted to today's transit.) Spectral observations during the transit helped scientists determine the composition of Venus' atmosphere, and one of the big scientific projects this time around will use the "Arc of Venus" to unravel more of the atmosphere's secrets.

The Hubble Space Telescope is being enlisted to watch the moon for changes in the characteristics of reflected sunlight due to Venus' passage. Such observations may blaze a trail for analyzing the atmospheres of alien planets. ESA's Venus Express and Japan's Hinode sun-watching satellite will also be on the case. "The most spectacular images and movies should come from Hinode’s Solar Optical Telescope, which has by far the highest resolution of any solar instrument in space," said Bernhard Fleck, ESA's Hinode project scientist.

Even if you don't see a single picture from the transit, you've got to appreciate the role that the event has played over the centuries in planetary science. These two videos tell you more about the history of transit observations:

Part 1: This video traces the early history behind observations of the transit of Venus.

10. Take a picture: Astrophotographers will be having a heyday. On the International Space Station, NASA astronaut Don Pettit is aiming to take the first pictures of the transit of Venus from outer space, while shooters on Earth will be trying to catch the space station crossing the sun's disk in parallel with Venus, as they did in 2004. NASA has set up a Flickr page for a citizen-science opportunity called the Venus Transit Observing Challenge, which should appeal to lots of amateur photographers. But if you go out to take a snapshot, do it safely and surely. Here's how.